Why the following xor operation is resulting the wrong answer?

Question

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> input = {2,3,4,5,4,3,7,2};
    int XOR = 0;

    for (auto& i : input)
    {
      XOR = XOR ^ i;
    }
    return XOR;
}

This returns 2, when it should return 5. Can anyone please explain why?

Looks like it should return 2. Did you try working through it mentally/on paper for each loop iteration and comparing that to debugging it one loop iteration at a time? — tinman, Jul 08 '23 at 23:33
The number pairs cancel each other out so you're left with `5 ^ 7 == 2` — Blastfurnace, Jul 08 '23 at 23:37

JeJo · Accepted Answer · 2023-07-09T00:19:34.287

[...] returns 2, when it should return 5. Can anyone please explain why?

Add some debug lines, and you will see what is happening inside. With the help of std::format you can even see the binary representation of each XOR and i directly in each iteration. https://gcc.godbolt.org/z/Te1786j5j

#include <format>  // std::format

int iter{ 1 };
for (const auto& i : input)
{
    std::cout << std::format("Iteration {} : XOR {:03b} ^ i {:03b} = ", iter++, XOR, i);
    // For binary representations -------------> ^^^^^^     ^^^^^^   
    XOR = XOR ^ i;
    std::cout << std::format("{:03b}\n", XOR);
}

Prints:

Iteration 1 : XOR 000 ^ i 010 = 010
Iteration 2 : XOR 010 ^ i 011 = 001
Iteration 3 : XOR 001 ^ i 100 = 101
Iteration 4 : XOR 101 ^ i 101 = 000
Iteration 5 : XOR 000 ^ i 100 = 100
Iteration 6 : XOR 100 ^ i 011 = 111
Iteration 7 : XOR 111 ^ i 111 = 000
Iteration 8 : XOR 000 ^ i 010 = 010

Now you see, it shouldn't be 5(i.e 101), rather 2 (i.e. 010).

Why the following xor operation is resulting the wrong answer?

1 Answers1